1. Field of Invention
The present invention generally relates to semiconductor processing. More particularly, the present invention relates to forming trench isolation structures during semiconductor device fabrication.
2. Discussion of the Related Art
With the ever-increasing number of semiconductor devices being built on a single chip, the art of isolating semiconductor devices has become an important aspect of modern semiconductor and integrated circuit technology. Improper isolation among devices will cause current leakage, which can consume a significant amount of power. Improper isolation can also result in defects such as noise margin degradation, voltage shift, and crosstalk.
Trench isolation is an isolation technique developed especially for a semiconductor chip with high integration. Shallow trench isolation (STI), in particular, is often used in the fabrication of integrated circuit devices to isolate active areas from one another. The trench regions are formed in the semiconductor substrate by recessing the substrate deep enough for isolation and filling with insulating material to provide the isolation among active devices or different well regions.
High density plasma chemical vapor deposition (HDP-CVD) of dielectric material has been previously used for STI gapfill. The HDP-CVD process includes a deposition component and a sputtering component to simultaneously deposit and etch the dielectric material in the same reaction, thereby allowing material to be deposited very densely and without voids.
Liner oxide layers have been used to prevent plasma damage to an underlying substrate from the HDP-CVD gapfill process. Such a method requires that the liner oxide layer be of sufficient thickness to prevent damage to underlying layers from the sputtering component of the HDP-CVD process. For example, the use of one layer of thick thermal oxide is known, but such a technique has caused corner effects such as gate wrap around and parasitic leakage. Furthermore, in the fabrication of flash memory devices, the use of only a single layer of thick thermal oxide will cause the tunnel oxide to be thicker at the edges of the active area.
The use of a thermal oxide layer in conjunction with a high temperature CVD oxide (HTO) is known to advantageously solve the aforementioned problems. However, this technique may cause grooving of the oxide in a subsequent etch step, as shown in FIG. 1. Oxide layers 2 and 4 form grooves 8 while bulk oxide layer 6 is substantially level along a top surface of the dielectric layers. Furthermore, such a technique requires three steps of oxide deposition: one thermal oxide layer formation; one HTO layer formation; and one gapfill oxide layer formation. Thus, such a process involves a relatively high thermal budget and long process time.
Therefore, what is needed is a trench isolation technique that protects the underlying layers and/or the substrate from plasma damage and also provides a flat and uniform surface level of the dielectric in the trench after an etch. Furthermore, a highly efficient trench isolation process in terms of process cycle time and thermal budget is desirable.
The present invention provides a method of forming trench isolation structures without dielectric grooving. A method of forming trench isolation structures with improved thermal budget and process cycle time is also provided.
According to one embodiment of the present invention, a method of forming an isolation trench is provided, comprising forming a plurality of stack structures over a substrate, etching a trench in the substrate between two of the stack structures, forming an oxide liner over the trench, and depositing a bulk oxide layer over the oxide liner to fill the trench. The oxide liner has a similar etch rate as the bulk oxide layer and in one example, the etch rate of the oxide liner is within about 10% of the etch rate of the bulk oxide layer. Furthermore, the method includes etching the oxide liner and the bulk oxide layer to form a dielectric top surface between the two stack structures, wherein the dielectric top surface is substantially planar and below a top surface of the two stack structures.
According to another embodiment of the present invention, a method of forming an isolation trench is provided, comprising forming a plurality of stack structures over a substrate, etching a trench in the substrate between two of the stack structures, growing a thermal oxide layer over the trench, and filling the trench using a high density plasma chemical vapor deposition (HDP-CVD) process. The HDP-CVD process includes depositing an in-situ oxide layer over the thermal oxide layer, and depositing a bulk oxide layer over the in-situ oxide layer, wherein the in-situ oxide layer has a similar etch rate as the bulk oxide layer. In one example, the etch rate of the in-situ oxide layer is within about 10% of the etch rate of the bulk oxide layer. The method further includes etching the thermal oxide layer, the in-situ oxide layer, and the bulk oxide layer to form a dielectric top surface between the two stack structures, wherein the dielectric top surface is substantially planar and below a top surface of the two stack structures.
According to another embodiment of the present invention, a trench isolation structure is provided, comprising two stack structures over a substrate, a trench in the substrate between the two stack structures, an oxide liner over the trench, and a bulk oxide layer over the oxide liner, wherein the etch rate of the oxide liner is within about 10% of the etch rate of the bulk oxide layer.
Advantageously, the present invention allows for a highly efficient trench isolation process in which a uniform trench dielectric is formed substantially level at a top surface without grooving after an etch step. Furthermore, the present invention allows for improved efficiency by providing for a lower thermal budget and a shorter process time.
These and other features and advantages of the present invention will be more readily apparent from the detailed description of the embodiments set forth below taken in conjunction with the accompanying drawings.